🚀 go-pugleaf

> I need a circuit that will provide variable speed control of a DC motor. The
> operator control should be via some device with a knob attached. Would this
> be a variable resister? The control should be roughly linear, let me explain.

I use the circuit below for model trains (I have left out the direction
switches):

                 3A rectifier		

                    |\ |
    +12-15V >-------| >|---------+---------------------+-----------+
		    |/ |         |                     |           |
				 |                    ---      ---------
				 |          3A         ^       | motor |
				 |       rectifier    / \      ---------
				 |                    ---          |
				 |                     |           |
				/             /--------+-----------+
				\            / 
				 \          /  (Emitter)
				  \       |< 
			1K Pot    /<------|   PNP, 3A General Purpose
				 /        |\       (WITH HEAT SINK!)
				/           \ 
				\            \
				 \            \
				  \            |
				  /            |
				 |             |
				 |             |
   Ground   <--------------------+-------------+

This gives a roughly linear relationship between knob position and motor RPM.
Matching the "halfway" points depends on what motor you are using.  If you
insist on the "halfway" criterion, or if you want STRICTLY linear operations,
you probably will have to follow Larry Lippman's advice.  However, you might
want to experiment with this first.

By the way, you didn't say how much current the motor takes -- for small "can"
motors, 500 mA is more than enough; for larger motors, 2-3A (or more!) could be
used.  You probably should experiment with an ammeter to see how much you're
using.  This design is good for most SMALL DC motors.

All parts are available from Radio Shack, but if you buy from them you're on
your own :-).

Make sure you heat sink the PNP transistor -- If you are pulling a couple of
amps through the motor at ~10 volts, the PNP will be dissipating 20 Watts!
This is plenty enough to get (VERY!) warm.

E-mail me if you have more questions.

Stu Bell
HP Colorado "Chips 'R' Us" Division

Disclaimer: The above circuit may have been generated by line noise.  HP
certainly knows nothing about it.  In fact, >I< may know nothing about it, or
what is said here.  Don't sue me, please!